Yam is the common name for some of the several Dioscorea species (Dioscorea sp.) within the genus Dioscorea, a member of the monocotyledonous family Dioscoreacea, and is a staple food in West Africa, Southeast Asia, and the Caribbean. Yam has very high nutritional value, for example, it is high in vitamin C, dietary fiber, vitamin B6, potassium, and manganese, which may all promote good health, while being low in saturated fat, sodium, and cholesterol.
Dioscorea has been studied for years for their medicinal effects. In the Chinese pharmacopoeia, the medicinal uses of Dioscorea rhizome are prescribed for indigestion, anorexia, diarrhea and diabetes. Recently several species are known to be pharmacologically active against hypertension, heart disease and other physical ailments (Chen, H. L., et al., Nutrition, 19: 646-651, 2003). Moreover, yam tuber mucilage was reported to exhibit antioxidant ability (Hou, W. C. et al., J. Agric. Food Chem. 49: 4956-4660, 2001).
In recent years, the medicinal effect of Dioscorea polysaccharides also has been studied and published. In the Journal of China Pharmaceutical University, 25(6):369-72, 1994, it was reported that Dioscorea polysaccharides decreased, in vitro, the NADPH-Vc induced and cysteine-Fe2+ induced malondialdehyde formation of brain, liver and kidney microsomes in rats, and scavenged superoxide radicals generated by the hypoxanthine/xanthine oxidase reaction system and Fenton reaction system. Therefore, Dioscorea polysaccharides seem to be used as an antioxidant and superoxide radical scavenger. In the Journal of Plant Resources and Environment, 5(2):5-8, 1996, it is illustrated that the content of polysaccharide and allantoin of Dioscorea tuber have a remarkable effect on lowering the sugar and lipid levels in blood.
The mucous membranes covering the aerodigestive and the urogenital tracts, as well as the eye conjunctiva, the inner ear and the ducts of all exocrine glands are endowed with powerful mechanical and chemical cleansing mechanisms that degrade and repel most foreign matter. In addition, a large and highly specialized innate and adaptive mucosal immune system protects these surfaces, and thereby also the body interior, against potential insults from the environment. In a healthy human adult, this local immune system contributes almost 80% of all immunocytes. These cells are accumulated in, or in transit between, various mucosa-associated lymphoid tissues (MALT), which together form the largest mammalian lymphoid organ system (Dahan, S. et al., Immunol. Rev. 215: 243-253, 2007).
As for the importance of the mucosal system, there is currently great interest in developing mucosal vaccines against a variety of microbial pathogens, for the following reasons: (1) the vast majority of infections occur at or enter the body from a mucosal surface; (2) mucosal vaccines would also carry less risk of transmitting the type of infections still associated with the use of injectable vaccines in several parts of the world, such as hepatitis B virus and HIV infections; (3) oral vaccine administration could lead to simplified manufacturing of vaccines, thereby increasing the potential for local vaccine production in developing countries (Holmgren, J. et al., Vaccine 21: Suppl 2:S89-S95, 2003).
However, to date, except for oral polio vaccine, there is still no good mucosal vaccine that has been developed. Oral tolerance is one of the characteristics of the mucosal system, but is also the biggest obstruction for the vaccine development (Eriksson, K. et al., Curr. Opin. Immunol. 5: 666-672, 2002). Cholera toxin and E. coli heat-labile enterotoxin are the best-studied and most potent mucosal adjuvants up to the present. Unfortunately, they are also highly toxic to humans (Holmgren, J. et al., supra). Accordingly, it is important to find a better mucosal adjuvant. The present invention satisfies this need.